How much floor space is there?
Like what was said and you commented on. the tires are too small and the axles too small to carry a car(2500#) and camper. Like said, I think it is built for smaller lighter vehicles?
Remember, your payload includes 120 gallons of water to begin with. Then you fill up the holding tanks and add more fresh water before dumping the holding tanks sometimes if your not near a dump station...they weight a lot. Propane for inside the tank. Then add all the things you put into a RV/camper. That don't leave that much extra carrying capacity.
I have said recently on here about living quarters trailers, they are heavy. 13k pounds is pretty heavy to be towing. Try taking that some place like a soft field somewhere...